1. Field of the Invention
The present invention relates to a slice level adjustment unit and an EPG data obtaining device using the same. In particular, the present invention relates to a slice level adjustment unit that can adjust a slice level correctly even if a ghost image or the like is generated.
2. Description of Related Art
Conventionally, delivery of electric program guide (hereinafter referred to as “EPG”) data is performed utilizing a vertical blanking interval (hereinafter referred to as VBI) or the like of a TV signal. First, an example of a form of EPG data transmission will be described in brief.
EPG data of one time is transmitted by a broadcasting station from a predetermined time for a few minutes approximately a few times a day. In addition, the same EPG data is transmitted repeatedly for three times or so continuously for a purpose of compensation for a reception error or the like. FIG. 8 shows contents of a TV signal that carries the EPG data. As shown in FIG. 8, the part in which the EPG data is coded (hereinafter referred to as an “EPG signal”) is positioned between a horizontal synchronization signal plus a burst signal and the next horizontal synchronization signal of the TV signal. Furthermore, transmission of the EPG data is performed in four lines (from the tenth to the thirteenth lines) in the vertical blanking interval in a field period.
In addition, the EPG signal is made up of parts of CRI (Clock Run In), FRC (Frame Code) and DATA from its head. The CRI is a part for synchronization with a bit clock, the FRC is a part concerning an identification code that is unique to each VBI standard, and the DATA is a part in which data concerning contents of EPG is coded.
Since the EPG signal is transmitted by a broadcasting station, it is a signal in which digital information described above is modulated. Therefore, a receiver side obtains the digital information one after another by comparing values of the EPG signal with a predetermined slice level using a sampling pulse, for example. Therefore, if the slice level is not set appropriately, a serious error may be generated in a stage of obtaining the digital information. In particular, if amplitude of the EPG signal fluctuates due to a noise such as a ghost image, the error is apt to occur and cause a serious problem.
In a conventional method to deal with this problem, a maximum value and a minimum value of the EPG signal during a predetermined period are detected, and an average value of them is calculated so that the slice level after that time is set to a value of the calculated result as shown in FIG. 8, for example. In addition, Japanese examined patent publication No. 08-032026 discloses a technique of adjusting the slice level as necessity so that text broadcasting data can be extracted correctly even if there is distortion in a received signal due to a noise such as a ghost image.
It is preferable that the adjustment of the slice level be performed in accordance with a state of the EPG signal that is currently received so as to correspond to a noise environment that is changing continuously, so that the slice level is adjusted to a level that is suitable for the current situation. In addition, it is preferable that the adjustment be performed as quickly as possible after starting the transmission of the EPG signal so that the EPG signal can be processed using the adjusted slice level at a stage as early as possible. Therefore, it is important to determine the timing of the adjustment.
Concerning this point, there is a method in which the adjustment of the slice level is performed constantly regardless of whether or not the EPG signal is received currently (hereinafter referred to as a “first method”). In this case, although the slice level is not adjusted appropriately during a period while the EPG signal is not received, there is no problem because the EPG signal is not coming actually. In addition, when reception of the EPG signal is started, the slice level is adjusted in accordance with a current situation automatically.
As another method, there is a method in which presence or absence of the EPG signal in the TV signal that is transmitted continuously is detected, and the adjustment of the slice level is started when coming of the EPG signal is detected (hereinafter referred to as a “second method”). In this case, amplitude of the TV signal at a position to be assigned to the EPG signal is monitored constantly. If the amplitude exceeds a constant value, it is determined that the EPG signal is received, so that the adjustment of the slice level is started.
According to the first method described above, it is not necessary to detect a time when the EPG signal comes, but the slice level is adjusted automatically when the EPG signal comes in accordance with the current situation. However, since the EPG signal is transmitted only a few times or so a day, so this method in which the slice level is adjusted constantly causes increase of an unnecessary load on a controller or the like and is very inefficient.
In addition, according to the second method described above, the load on the controller or the like can be reduced largely compared with the first method because the slice level is adjusted only when the EPG signal comes. However, according to this method, the EPG signal may not be detected correctly in a case where a certain noise is mixed into the signal. In addition, if the signal waveform when the EPG signal comes is similar to the signal waveform when the EPG signal does not come, it may be difficult to detect the coming itself of the EPG signal.